Various types of force sensors have been used to control motions of robots and industrial machines. Also, a downsized force sensor has been incorporated as a man-machine interface of an input device for electronics. In order to reduce dimensions and costs, a force sensor to be used in the above applications is required to be as simple as possible in structure and also to independently detect force for each coordinate axis in a three dimensional space.
At present, generally used multi-axis force sensors are categorized into such types that a specific directional component of force exerted on a mechanical structure portion is detected as displacement occurring at a specific site and such types that the specific directional component of force is detected as mechanical strain occurring at a specific site. A force sensor which is of a capacitive element type is a representative sensor of the former which is of a displacement detection type. This force sensor has a capacitive element constituted with a pair of electrodes and detects displacement occurring at one of the electrodes by the force exerted on the basis of a capacitance value of the capacitive element. A multi-axis force sensor which is of capacitive element type has been disclosed, for example, in Patent Document 1 (Patent Document 2 which is an English version thereof) and Patent Document 3 (Patent Document 4 which is an English version thereof) given below.
On the other hand, a strain gauge type force sensor is a representative sensor of the latter which is of a mechanical strain detection type. This force sensor detects a mechanical strain resulting from the force exerted as a change in electrical resistance of strain gauges, etc. The strain gauge-type multi-axis force sensor has been disclosed, for example, in Patent Document 5 (Patent Document 6 which is an English version thereof) given below.
However, in any of the multi-axis force sensors disclosed in each of the above-described Patent Documents, a mechanical structure portion thereof is inevitably made thick. Thereby, it is difficult to make thin the sensor in its entirety. On the other hand, in the fields of robots, industrial machines, input devices for electronics, etc., it is desired to develop a thinner-type force sensor. Thus, in Patent Document 7, there has been proposed a force sensor in which an annular member is deformed by exertion of force to detect displacement of each portion resulting from the deformation by using a capacitive element. The force sensor which has been disclosed in Patent Document 7 (which is termed “a force sensor of the prior application” in the present application) is simplified in structure, thereby having a structure suitable for being made thinner.